The aim of this project is to study the effects of oral intake of lead and methyl mercury on auditory and vestibular function in juvenile squirrel monkeys. Subjects will be given daily doses of red lead solution (500 to 700 mg/kg) or methyl mercury hydroxide (0.3 mg/kg). Under pentobarbital anesthesia, experimental animals will be fitted with permanently implanted dural electrodes and depth electrodes with tips located in the auditory nerve/cochlear nucleus region. An untreated control group will be implanted with identical sets of electrodes. At regularly spaced intervals of time, auditory nerve and brain input-output curves as well as visual detection levels (absolute thresholds) will be obtained from computer-averaged evoked responses (AER). Stimuli will be presented through a loudspeaker and will consist of 50-ms duration tone bursts from 125 through 16,000 Hz. Vestibular function will be assessed using standard clinical techniques by recording positional and caloric-induced nystagmus (ENGs) before, during and following the poisoned state. After obtaining all functional data, the animals will be sacrificed by intravital perfusion with saline solution followed by 10% formol. Temporal bones will be dissected out and sections will be prepared for light microscopic evaluation of inner ear morphological changes.